The present invention relates to an acoustic polarimeter for determining the acoustic birefringence of a solid with parallel faces, for one direction of the acoustic wave perpendicular to the parallel faces.
The method of analyzing stresses by optical birefringence induced in a transparent solid is currently used under the name of "photoelasticimetry" or measurement of photoelasticity. This method may be transposed to non transparent solids such as metals with acousto-elasticimetry based on the acoustic birefringence of the solids.
Acoustic birefringence is the consequence of the elastic anisotropy presented by most solids, whether it is structural in origin or induced by stresses or the two combined. As in optics, this acoustic birefringence is characterized by two main slow and fast axes perpendicular to the direction of propagation and related to the speed of the transverse acoustic waves polarized parallel to these axes. Two parameters are sufficient for determining it: the orientation .theta. of the fast axis with respect to a reference direction and the phase-shift .phi. between the slow and fast waves after passing through the solid.
It is known from U.S. Pat. No. 4,138,894 to determine the parameters .phi. and .theta. of a plate with parallel faces by disposing two transducers close to the input face of this plate, the transducers transmitting or receiving waves polarized at 90.degree. with respect to each other at the angular frequency .omega.. The transmitted waves are modulated at a so-called low frequency angular frequency .OMEGA., so as to obtain a polarization outputted by the assembly of the two transducers which rotates at the angular frequency .OMEGA.. The signals received by the two transducers are processed in circuits which also receive the signals at the angular frequency .omega. shifted in phase by .phi. by a manual phase-shifter. By adjusting this phase-shifter the value of .phi. may be obtained by display, .theta. being obtained by synchronous detection of a signal at the angular frequency .OMEGA..
The device has the disadvantage of requiring adjustment of the phase-shifter to a predetermined value which requires accurate calibration. Moreover, the value of .phi. is not obtained directly.
The polarimeter of the invention has the advantage with respect to the prior art of supplying directly without adjustment of .phi. the parameters .phi. and .theta. and it thus facilitates use of acousto-elasticimetry in mechanics, whereas the known methods could only be applied in specialized laboratories.
According to the main feature of the invention, this is an acoustic polarimeter for determining the birefringence parameters .theta. and .phi. of a plate with parallel faces, this plate being fixed to the endmost face of an elongated isotropic body with parallel faces, formed from a material propagating acoustic waves of frequency .omega./2.pi. while on the other endmost face of this body is placed a pair of transducers having their linear transverse axes of polarization at 90.degree. with respect to each other, the electric signals applied to the first and second transducers being respectively of the form cos .OMEGA.t cos .omega.t and sin .OMEGA.t cos .omega.t, where .OMEGA./2.pi. is a low frequency (.OMEGA.&lt;&lt;.omega.), signals x.sub.1 and y.sub.1 being received by the two transducers after reflection from the rear face of the birefringent plate and these signals being multiplied by the signal cos (.omega.t-.phi.) supplied by a phase-shifter, these signals processed and filtered supplying the signals X and Y; wherein the signals X and Y are both multiplied by cos .OMEGA.t and sin .OMEGA.t and a differential amplifier supplies the alternating signal A such that A=X sin .OMEGA.t-Y cos .OMEGA.t and a second differential amplifier supplies the alternating signal B so that: B=X cos .OMEGA.t-Y sin .OMEGA.t measured respectively by AC voltmeters and wherein means allow, by acting successively on the phase-shifter, signals A and B to be cancelled out and so signal A.sub.B, value of the amplitude of A, A.sub.1 for B=0 and signal B.sub.A, value of the amplitude of B.sub.1 for A=0 to be obtained and wherein .phi. is determined from these values by the relationship tan .phi.=A.sub.B /B.sub.A, .theta. being obtained by a phasemeter receiving the signal A and a signal sin 2.OMEGA.t.